Crack bridging by uncracked ligaments during fatigue-crack growth in SiC-reinforced aluminum-alloy composites
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I.
INTRODUCTION
R E C E N T studies in a wide range of metallic and nonmetallic materials have demonstrated the role of cracktip shielding in enhancing toughness, or more generally in impeding crack advance, by locally reducing the crack driving force actually experienced at the crack tip; notable examples are transformation-toughening in ceramics, robber-toughening in polymers, and fatigue-crack closure in metals, as reviewed in References 1 and 2. In certain materials, particularly ceramic composites, a prominent shielding mechanism occurs by bridging between the crack faces; t3-~71 such bridges can be generated by the presence of brittle fibers, t3-81 ductile particles, [9-121or unbroken ligaments in the wake of the crack tip.j13-171 The dominant contribution to toughening from bridging reinforcements can be expressed simply as the product of the volume fraction of the bridging phase with the area under the stress/strain curve, i.e., in terms of the yield strength, o-y, radius Ry, and volume fraction Vl of the reinforcement phase: w]
Gc =
CO-yRfVf
[1]
where C depends on the ductility of the reinforcement phase and the extent of interface debonding. In brittle fiber-reinforced ceramic-matrix composites, such as SiC fibers in alumina where the fibers are sufficiently strong and the fiber/matrix interface sufficiently weak, preferential failure in the matrix can leave intact fibers spanning the crack for some distance behind the crack tip; as the crack opens, fiber motion then is restrained, for example, by friction in the interface, t~'3-sl With ductile-phase reinforcements, such as rubber in polymers or aluminum particles in alumina, particles in the crack path can similarly act as bridges and contribute to the toughness by exhibiting extensive plastic stretching in the crack w a k e . ]9-121 In either case, the reinforcement phase, provided it remains unbroken and is intercepted by the crack, can act as a series of springs which restrain crack opening and thereby shield the crack tip from the
applied far-field loading, resulting in lower, yet cracksize dependent, growth-rate behavior, t3-8~ In metallic materials, similar bridging effects are developed in aluminum-alloy laminates reinforced with epoxy-resin sheets impregnated with unidirectional aramid fibers (ARALL ~* Laminates), where the fiber/epoxy *ARALL Laminate is a registered trademark of the Aluminum Company of America.
interfaces now are weak enough to permit controlled delamination and, thus, bridging of unbroken fibers across the crack. [18,191However, in most metal-matrix composites, such as aluminum alloys discontinuously reinforced with SiC fibers (or whiskers or particles), the reinforcement phase invariably fractures due to its strong interface with the matrix, with the result that particle or fiber-bridging essentially is insignificant. [~7,2~ Recently, however, studies on fatigue-crack growth in aluminum alloy/SiC-particulate composites (A1/SiCp) have revealed a different mechanism of bridging, induced by the presence of uncracked ligam
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